1. Field of the Invention
A present invention relates to a wireless LAN communication system, and more particularly to communication priority control in a CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) system.
2. Related Background Art
In a CSMA/CA system, a terminal performs virtual carrier sensing, to be described below, for a random time period prior to communication to check whether another terminal is communicating with a base station, and when another terminal is communicating, waits for the communication to end and then performs actual packet transmission. At this time, all terminals have equal transmission rights. In virtual carrier sensing, a random number is generated within a prescribed CW (Contention Window) range after a channel has been idle for an IFS (Inter Frame Space) time period, and a random time period is determined on the basis of the value of the random number. Back-off control, to be described below, is then performed during the random time period. In back-off control, the calculated random number value is set as an initial value, the value is reduced over time, and when the value reaches zero, actual packet transmission is performed. Here, IFS is a certain fixed time period prescribed by the wireless LAN standard IEEE802.11 for performing idle detection prior to transmission, and CW is the maximum random number value that can be obtained during back-off, which serves as a required parameter for realizing user-multiplexing. In IEEE802.11, a minimum value CWmin and a maximum value CWmax of CW are prescribed. Upon the initial transmission, back-off is performed by calculating a random number value using the value of CWmin, and upon every retransmission, CW is doubled. Note that CWmax is the upper limit value of CW. By means of this randomness-dependent back-off, communication can be performed with a plurality of terminals sharing an identical channel. However, with this system, a plurality of terminals may perform packet transmission simultaneously. When a plurality of terminals perform packet transmission simultaneously, a packet collision occurs such that none of the terminals can receive packets correctly, and as a result, the communication quality deteriorates. A particularly striking deterioration in the communication quality occurs in a real time application such as VoIP.
EDCA (Enhanced Distributed Channel Access), which is prescribed by IEEE802.11e and disclosed in the document “Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications: Amendment 7: Medium Access Control (MAC) Quality of Service (QoS) Enhancements”, exists as prior art relating to priority control in this type of communication system. In EDCA, four priority types are attached to packets, and parameters relating to the transmission wait time, such as the aforementioned IFS, CWmin, and CWmax, are set to be short with respect to high-priority packets. Thus, packet transmission rights are allocated preferentially. In so doing, relative priority control can be performed in a communication environment in which various applications, such as voice applications, data applications, and so on, coexist.
However, conventional EDCA is merely a technique for attaching an order of relative transmission precedence to a plurality of packets having different set priorities, and not for ensuring quality among and attaching priority to a plurality of terminals that transmit packets having the same set priority. Accordingly, packet collisions occurring when a plurality of terminals transmit packets having the same priority at the same time cannot be suppressed. When a packet collision occurs, retransmission is delayed even if successful, and when retransmission fails, packet loss occurs. Furthermore, when a plurality of terminals perform back-off to obtain packet transmission rights, another terminal from which the transmission rights are obtained faces an increased transmission wait time, or in other words a delay, to re-obtain transmission rights. The probability of a packet collision increases as the number of terminals rises, and the probability of delays and packet loss increases accordingly. A particularly great deterioration in quality may occur when delays and packet loss occur in a real time application such as VoIP (Voice over Internet Protocol).
In a different approach to avoiding packet collisions, a wireless base station for controlling terminals in which packets are generated periodically, as in VoIP, performs transmission timing scheduling on the terminals. In particular, Japanese Unexamined Patent Application Publication 2005-252627 describes a technique that enables scheduling to be performed when terminals having different packet generation periods coexist.
However, the technique described in Japanese Unexamined Patent Application Publication 2005-252627 is merely a method of having a wireless base station schedule the transmission timing of a wireless communication terminal, and the wireless communication terminal itself is not capable of scheduling the transmission timing in an autonomous distributed manner. Furthermore, although the technique described in Japanese Unexamined Patent Application Publication 2005-252627 is a protocol according to which scheduling can only be performed when the wireless base station recognizes the packet generation period of all of the wireless communication terminals, a method of ensuring that the wireless base station recognizes the packet generation period of all of the wireless communication terminals is not realized. Moreover, a method of allocating a specific transmission time period to each terminal within a predetermined scheduling period is not specified. Also, the transmission time period has a fixed length regardless of the terminal, and differences in the time required for each terminal to perform a transmission/reception procedure corresponding to varying transmission rates and packet lengths are not taken into account. Differences in the modulation system that can be supported by each terminal are not taken into account either.